1. Field of the Invention
This invention relates to a magneto-optical recording medium taking advantage of the magneto-optical effect to record or reproduce the information by the laser beam or the like. More particularly, it relates to a magnet-optical recording medium making stable maintenance of the magneto-optical characteristics possible while suppressing corrosion or eroded pit formation.
2. Description of the Prior Art
In recent years, as the rewritable high density recording system, attention has been generally directed to a magneto-optical recording system recording and/or reproducing the informantion with, for example, a semiconductor laser light beam.
As typical of the recording material for the magneto-optical recording system, an amorphous alloy consisting of a combination of rare earth elements, such as Gd, Tb or Dy and transition metal elements, such as Fe, Co or Ni, has been employed. However, the rare earth elements or Fe making up the amorphous alloy tend to be oxidized readily and are easily combined with oxygen in air to form oxides. When the oxidation proceeds to the stage of corrosion or eroded pit formation, signal dropout will be caused. On the other hand, when the rare earth metals undergoes selective oxidation, the C/N ratio will be deteriorated with the decrease in coercivity and the residual magnetic Kerr rotation angle. These problems cannot be avoided iso long as the rare earth metals are used.
Such corrosion or eroded pit formation can be prevented from occurring by adding inert elements such as Pt, Pd or Co or elements capable of forming passivated films, such as Ti, Cr or Al, to the amorphous alloy thin film, and the favorable effect of such addition is confirmed in the case of a layer of a larger thickness. However, Unfortunately, the use of the aforementioned addition elements results frequently in the lowering of the magnetic Kerr rotation angle, while the desired effect is not attained with the thickness of the layer of not larger than 500 .ANG., such that a protective film or films need be employed simultaneously.
For practical application of the magneto-optical recording medium, it is essential not only to increase the aforementioned magnetic Kerr rotation angle, but to increase coercivity and the rectangular ratio of the magnetic Kerr loop. The rectangular ratio herein means the ratio between the residual magnetic Kerr rotation angle .theta..sub.k.sup.r and the saturation magnetic Kerr rotation angle .theta..sub.k.sup.s. The value of the ratio closer to unity is more favorable. Recent researches have revealed that the rectangular ratio is intimately related with the monoaxial anisotropic energy of the recording layer, such that, the closer the ratio to unity, the sharper the form of the write bit owing to the increased monoaxial anisotropic energy or to the decreased dispersion of the anisotropic energies, resulting in the improved C/N ratio at read-out information.
On the other hand, the Co-Pt or Co-Pd type material, employing noble metals such as Pt or Pd in place of the rare earth elements, is superior in corrosion resistance, and expected to be used as, for example, the recording material for the magnetic recording medium.
However, in these Co-Pt or Co-Pd type materials, as reported in the "Journal of Magnetism and Magnetic Materials", Vol 66, pages 351 to 355, 1987, only perpendicular magnetic anisotropy has been found to exist for the layer thickness as large as about 2000 .ANG., while it has not been tried to apply the material to the magneto-optical recording medium.